Statutory regulations require that all exhaust-gas relevant components of a motor vehicle be diagnosed. Faults which lead to the exhaust-gas limit value to be exceeded by a factor of 1.5 must be detected in running operation of the vehicle and, for example, indicated by switching on a fault lamp. When the diagnostic lamp illuminates, the driver is instructed to immediately go to a service facility and have the fault corrected. An unnecessary visit to the service facility as a consequence of an erroneously illuminated fault lamp is to be avoided.
The statutes permit diagnostic functions to be switched off or permit a delayed entry of a fault when the environmental conditions make an erroneous fault detection probable, for example, when the air pressure is too low at high elevations.
U.S. Pat. No. 5,158,059 discloses a method in this context wherein the detection or registration of faults is suppressed when the tank is empty. This measure is supported by the possibility that, for an almost empty tank, fuel as well as air reaches the fuel supply of the engine. As a consequence, the precision of the fuel metering deteriorates. Considerable mixture leaning takes place which, in turn, can lead to a significant enrichment reaction in a closed air/fuel ratio control loop. The mixture deviations, especially the leaning of the fuel mixture, can cause combustion misfires. Depending upon the construction of the fuel tank, several kilometers can still be travelled from the first occurrence of the leaning of the fuel mixture until the vehicle comes to final standstill. This effect affects a series of diagnostic functions which react with fault announcements. These fault announcements do not result from a defective component. For this reason, the fault announcements should not lead to driving the fault lamp. The diagnostic functions which are affected are, for example, the detection of combustion misfires, the diagnosis of the fuel supply system, the diagnosis of the .lambda.-control by means of a .lambda.-probe rearward of the catalytic converter, the diagnosis of the catalytic converter and the diagnosis of the .lambda.-probe.